Method for hydrogenating liquid organic compounds

ABSTRACT

Liquid organic compounds are hydrogenated by a process in which the hydrogen present in the reactor contains proportions of at least one gas which is inert in the hydrogenation reaction.

This application is a 371 of PCT/EP02/00766 filed Jan. 25, 2002.

The present invention relates to a process for hydrogenating liquidorganic compounds, in particular for hydrogenating nitro compounds togive amines.

The hydrogenation of liquid organic compounds is one of the mostfrequent processes in the chemical industry. An industrially importanthydrogenation reaction is the hydrogenation of nitro compounds to givethe corresponding amines.

In hydrogenations of this type, the hydrogenation of nitroaromatics toaromatic amines is particularly important. Aromatic amines can be widelyused. For example, aniline is the starting material for many organicsyntheses. Tolylenediamine, frequently referred to as TDA, can bereacted with phosgene to give tolylene diisocyanate, one of theisocyanates most frequently used for the preparation of polyurethanes.

The catalytic hydrogenation of liquid starting materials is carried outin industry predominantly using a finely suspended catalyst or in afixed bed. The reactors used may be autoclaves, loop reactors, stirredkettles, bubble columns or reactor cascades. Such processes aredescribed, for example, in EP-A-263 935, EP-A-634 391, EP-A-124 010 orWO 00/35852.

Since the solubility of hydrogen in the liquid phase is generally verylow, the processes known in industry are operated under pressure inorder to increase the saturation concentrations of the hydrogen. Thegaseous hydrogen is frequently introduced at the bottom of the reactorsand dispersed by means of stirrers, nozzles or during flow throughpackings, in order to achieve a large specific exchange surface betweenthe gaseous and the liquid phase.

However, the disadvantage is that the system comprising hydrogenationbath and hydrogen exhibits strong coalescence, i.e. relatively largebubbles are formed again immediately after leaving the dispersing zoneand substantially reduce the specific exchange surfaces and hence themass transfer. This behavior is in contrast to the prevailing opinionthat coalescence is predominantly inhibited in a multimaterial mixture,such as the hydrogenation bath, and the properties of the gas have anegligible influence on the coalescence behavior.

The hydrogen which is used for the hydrogenation and is usually producedindustrially in synthesis gas plants generally has a purity of more than99.95% by volume. This is intended to ensure that a very small amount ofinert gases are entrained into the hydrogenation reactors.

An advantage of the procedure using very pure hydrogen is thecontinuously high hydrogen partial pressure in the gas phase of thereactor, which facilitates the transfer from the gas phase into theliquid hydrogenation bath.

Furthermore, it is possible to ensure that only a small amount of thehydrogen fed to the reactor has to be discharged from the reactor aswaste gas.

In general, in the processes customary in industry, from about 1 to 2%of the hydrogen introduced into the reactor are discharged as waste gasso that the gaseous byproducts and the small amounts of inert substancesare removed from the reaction mixture. However, as a result of this, thepurity of hydrogen in the waste gas stream generally does not fall below98% by volume.

In the prior art processes, the high partial pressure in the reactorcannot be utilized because of the coalescence described above at thehigh purity of the hydrogen. The inadequate exchange surface betweenliquid and gaseous phases therefore results in increased aging of thecatalyst and insufficient selectivity of the reaction, since thesaturation concentrations of the hydrogen in the hydrogenation bathcannot be reached.

It is an object of the present invention to provide a process forhydrogenating liquid organic compounds, in particular for hydrogenatingnitro compounds to amines, in which optimum mass transfer between thehydrogen and the hydrogenation bath takes place, the aging of thecatalyst is suppressed and the selectivity of the reaction is increased.

We have found that this object is achieved and that, surprisingly, inthe case of a mixture of the hydrogen used for the hydrogenation withproportions of at least one gas which is inert in the hydrogenationreaction, there is greater inhibition of coalescence and the masstransfer of the hydrogenation reactor can be substantially increased.

The present invention accordingly relates to a process for hydrogenatingliquid organic compounds,in particular for the preparation of amines byhydrogenating the corresponding nitro compounds, wherein the hydrogenused contains proporitions of at least one gas which is inert in thehydrogenation reaction.

The proportion of the gas which is inert in the hydrogenation reactionin the gas phase of the reaction is preferably from 3 to 50% by volume,based on the amount of gas in the reaction mixture. In principle, it isalso possible to work outside these limits. In the case of contentsbelow this range, however, troublesome coalescence of the gas in theliquid phase still occurs. In the case of contents above this range, theconcentration of dissolved hydrogen in the hydrogenation bath maydecrease owing to the low partial pressure of the hydrogen, in spite ofthe larger exchange surface, with the result that the yield of thereaction decreases. The content of inert gases is preferably from 3 to30, particularly preferably from 5 to 20, % by volume, based in eachcase on the amount of gas in the reaction mixture.

The gases which are inert in the hydrogenation reaction must be heavierthan hydrogen. In principle, all substances which are gaseous under thehydrogenation conditions and are inert in the reaction can be used forthe novel process. Examples of possible gases are nitrogen, noble gases,in particular neon, argon or krypton, ammonia, lower saturatedhydrocarbons, in particular methane, ethane, propane or butane, andcarbon dioxide. Nitrogen is of most importance here since it can behandled safely and without problems and is generally available insufficient quantities in industrial plants. Under certain reactionconditions, steam may also act as the gas which is inert in thehydrogenation reaction. Some of the water forms during thehydrogenation, but water may also enter the reactor with the reactioncomponents or as a solvent. In principle, however, preferred inertsubstances are those which cannot condense under the conditionsprevailing in the reactor, in order to avoid pressure variations orvarying compositions of the gas phase.

The gases which are inert in the hydrogenation reaction can be fed tothe reactor as a mixture with the hydrogen. However, it is also possibleto feed it in a separate stream to the reactor.

In a further embodiment of the invention, the concentration of the gaseswhich are inert in the hydrogenation reaction is established solely byregulating the waste gas stream. By throttling the waste gas stream fromthe reactor, the concentration in the reactor of the gases which areinert in the hydrogenation reaction can be established within thepreferred range.

The generally used hydrogenation reactors, such as autoclaves, bubblecolumns, stirred kettles, loop reactors or fixed-bed reactors, generallyprovide good back-mixing on the gas side through specific gas recyclingor through the reintroduction (redispersion) of gas from the gas phaseby means of driving jets or stirrers. The waste gas stream, which as arule is removed directly from the gas phase of the reactor, willtherefore generally have the same composition as the gas circulating inthe reactor.

Owing to the substantially complete back-mixing of the gas phase in thereactor, the desired concentration of hydrogen in the reactor can beestablished by specifying the amount of gas in this embodiment of thenovel process. If the concentration of the fresh hydrogen is 99.95% byvolume, the proportion of inert gases in the gas phase of the reactor isat least 5% by volume, based on the gas phase of the reactor, at adischarge rate of 1%, based on the fresh gas. If a discharge is reducedto 0.25%, based on the fresh gas, it is even possible for at least 20%by volume of inert gases to accumulate in the reactor. As stated above,substantially higher proportions of inert gases are however no longerexpedient, owing to an excessive decrease in the hydrogen partial,pressure.

When regulating the concentration of the hydrogen in the reactionmixture, by establishing the amount of waste gas, the lower limit of theconcentration of the hydrogen fed in should therefore be 98% by volume.In this case, with a discharge rate of 10%, based on the fresh gas, aproportion of about 20% by volume of inert gas can be established in thereactor. At lower discharge rates, the proportion of inert gas increasesabove the particularly preferred values; at higher discharge rates, theprocess may become uneconomical since excessively large amounts ofhydrogen have to be removed as waste gas. In principle, the novelproportion of inert gas in the gas phase of the reactor can also beestablished when the fresh hydrogen has purities of more than 99.95% byvolume. However, the amount of waste gas or the discharge rate must thenbe greatly reduced. Regulation of such small waste gas streams may beproblematic in practice. In this way, however, a proportion of 5% byvolume of inert gas in the gas phase of the reactor can be establishedat an exemplary purity of the fresh hydrogen of 99.99% by volume byreducing the discharge rate to 0.2%.

In order to achieve the desired concentration of the hydrogen, it isalso possible in principle to close the waste gas valve completely andto open it discontinuously only when a specific proportion of inert gasis exceeded.

The novel process is particularly advantageous in the case ofhydrogenations in which water of reaction is produced and which areoperated at below 150° C.

The advantage of a hydrogenation at above 150° C. is a higher vaporpressure of the water of reaction produced in the hydrogenation, whichwater then likewise acts as a gas which is inert in the hydrogenationreaction, with the result that the accumulation of other inertsubstances in the reactor can be reduced or is no longer necessary.However, a hydrogenation at above 150° C. has disadvantages, for exampleaccelerated catalyst aging and a higher level of byproduct formation.Temperatures above 150° C. are therefore not preferred for the novelprocess.

The fresh hydrogen can be fed into the gas phase of the reactor. This isadvantageous particularly in the case of flow reactors, as described,for example, in EP 634 391 or WO 00/35852. In this case, however, ashort-circuit with the waste gas line must be ruled out.

The fresh hydrogen is preferably metered into the liquid phase of thereactor. The introduction can be effected by means of known meteringelements. Possibilities include, for example, the introduction via oneor more ring lines in the liquid phase, one or more inlet pipes in theliquid phase, in particular at the bottom of the reactor, or, in thecase of stirred kettles, via a hollow-shaft stirrer.

The waste gas is generally removed at the top of the reactor, via thegas phase. As stated above, it must be ensured that no short circuit tothe fresh hydrogen occurs during removal of the waste gas, in order toavoid additional losses of hydrogen.

The novel process can be used in principle for all hydrogenations oforganic compounds which are liquid under the reaction conditions.Examples of these are the hydrogenation of benzene to cyclohexane, ofbutynediol to butanediol and of oxo aldehydes to the oxo alcohols. Nowater of reaction is formed in any of these processes, making thepresence of inert gases particularly important. The novel process can beparticularly advantageously used for the preparation of amines from thecorresponding nitro compounds, in particular of aromatic amines from thecorresponding aromatic nitro compounds.

Aromatic nitro compounds having one or more nitro groups and 6 to 18carbon atoms, for example nitrobenzenes, such as o-, m- andp-nitrobenzene and 1,3-dinitrobenzene, nitrotoluenes, such as 2,4- and2,6-dinitrotoluene and 2,4,6-trinitrotoluene, nitroxylols, such as1,2-dimethyl-3-, 1,2-dimethyl-4-, 1,4-dimethyl-2-, 1,3-dimethyl-2-,2,4-dimethyl-1- and 1,3-dimethyl-5-nitrobenzene, nitronaphthalenes, suchas 1- and 2-nitronaphthalene and 1,5- and 1,8-dinitronaphthalene,chloronitrobenzenes, such as 2-chloro-1,3- and1-chloro-2,4-dinitrobenzene, o-, m- and p-chloronitrobenzene and1,2-dichloro-4-, 1,4-dichloro-2-, 2,4-dichloro-1- and1,2-dichloro-3-nitrobenzene, chloronitrotoluenes, such as 4-chloro-2-,4-chloro-3-, 2-chloro-4- and 2-chloro-6-nitrotoluene, nitroanilines,such as o-, m- and p-nitroaniline, nitroalcohols, such astris(hydroxymethyl)nitromethane, 2-nitro-2-methyl- and2-nitro-2-ethyl-1,3-propanediol, 2-nitro-1-butanol and2-nitro-2-methyl-1-propanol, and any desired mixtures of two or more ofsaid nitro compounds are preferably used in the novel process.

Aromatic nitro compounds, preferably mononitrobenzene,methylnitrobenzene or methylnitrotoluene, and in particular2,4-dinitrotoluene or its industrial mixtures with 2,6-dinitrotoluene,these mixtures preferably comprising up to 35% by weight, based on thetotal mixture, of 2,6-dinitrotoluene with from 1 to 4 percent of vicinaldinitrotoluene and from 0.5 to 1.5% of 2,5- and 3,5-dinitrotoluene, arepreferably hydrogenated to the corresponding amines by the novelprocess. The novel process can be advantageously used in particular inthe hydrogenation of dinitrotoluene isomers to the correspondingtolylenediamine derivatives (TDA).

The hydrogenation of aromatic amines can be carried out in the absenceof a solvent or in solution. The solvents used are the substancescustomary for this purpose, in particular lower alcohols, preferablyethanol.

The novel hydrogenation is usually carried out in the presence ofcatalysts. Catalysts which may be used are the conventional and knownhydrogenation catalysts.

Examples of these are metals of subgroup VIII of the Periodic Table ofthe Elements, which metals can be applied to support materials, such asactive carbon or oxides of aluminum, of silicon or of other materials.Raney nickel and/or supported catalysts based on nickel, palladiumand/or platinum are preferably used. It is also possible in principle touse homogeneous catalysts.

The novel process using heterogeneous catalysts can be carried out bythe fixed-bed or suspension procedure. The fixed-bed procedure can becarried out by the liquid-phase or trickle-bed procedure.

In the suspension procedure, heterogeneous catalysts are likewise used.The preferred hydrogenation of nitro compounds to amines is likewisegenerally carried out in the presence of heterogeneous catalysts. Theheterogeneous catalysts are generally used in a finely divided state andare suspended in finely divided form in the reaction suspension.Reactors used for the hydrogenation in suspension are in particular loopapparatuses, such as jet loops or propeller loops, stirred kettles,which may also be equipped as stirred kettle cascades, bubble columns orair-lift reactors.

The novel process is generally carried out under the reaction conditionscustomary for the specific reaction. Thus, the conversion of aromaticnitro compounds into aromatic amines is usually carried out at from 5 to100, preferably from 10 to 50, bar and from 80 to 160° C., preferablyfrom 80 to 150° C., in particular from 100 to 150° C.

In the novel process, the exchange surface between gas phase and liquidphase can, surprisingly, be substantially increased. The larger exchangesurface makes it possible always to operate close to the saturationconcentration of the hydrogen in the hydrogenation bath. Consequently,the probability that the free sites on the catalyst surface will beoccupied by hydrogen molecules increases. This leads in turn to anincrease in the yield and the selectivity of the process.

The novel process can be used without problems and without additionalconversions in all existing hydrogenation reactors. The inert gasespreferred for the novel process, in particular nitrogen, are availablein sufficient quantitity at virtually all production locations.

A further advantage of the novel process is that expensive purificationof the hydrogen used is no longer necessary. It is sufficient to removefrom the hydrogen those components which act as a catalyst poison orwhich may lead to secondary reactions. This permits more economicalhydrogen production.

The examples which follow illustrate the invention.

EXAMPLE 1 (COMPARISON)

A cylindrical reactor having an external circulation, a baffle plate inthe lower reactor part and a concentric dip tube, as described inexample 1 in WO 00/35852 was used. The reaction volume of the reactorwas 0.05 m³. The reactor was provided with 36 field tubes which wereconnected in parallel and altogether corresponded to a cooling area ofabout 2.5 m². The amount of cooling water fed into the field tubes was 1m³/h and the temperature of the cooling water fed into the field tubeswas 30° C.

By means of a high-pressure pump, 40.3 kg/h of a dinitrotoluene melt,consisting of 80 parts by weight of 2,4-dinitrotoluene and 20 parts of2,6-dinitrotoluene, was sprayed at 120° C. into a fast-flowing mixtureof about 62 parts by weight of a corresponding diaminotoluene mixture,36 parts by weight of water and 2 parts by weight of a finely dividednickel hydrogenation catalyst. By simultaneous introduction of 30 m³(S.T.P.)/h of hydrogen, a pressure of 25 bar was maintained in thereactor. In order to maintain the loop flow, a volume flow of 2.6 m³/hwas circulated in the external product circulation. A pressure of about3 bar prevailed in the reaction nozzle, and the power supplied was 5kW/m³. The reaction took place under virtually isothermal conditionssince the resulting heat of reaction was removed at the place of itsformation. The maximum reaction temperature in the lower third of thereactor was 122° C. 26.7 kg/h of a corresponding diaminotoluene mixtureand 15.8 kg/h of water were removed simultaneously and continuously fromthe reactor with retention of the catalyst, which corresponded to aspace-time yield of 580 kg of amine mixture/m³*h.

The purity of the hydrogen supplied was 99.99999% by volume at adischarge rate of from about 1 to 2%. The waste gas was separated withthe aid of a condenser from the condensable fractions, which containedsubstantially steam, and was fed to the flare with a purity of over99.5% by volume of hydrogen. No additional inert gases were fed in. Thehydrogenation was operated at 120° C. and 26 bar absolute pressure. Theamount by weight of water in the hydrogenation bath was about 35%.

The gas phase in the reactor was composed virtually exclusively of thesteam from the water of reaction and the hydrogen. Thus, a total gasphase density of about 2.5 kg/m³ can be calculated. On introduction ofgas into the reactor with the aid of the nozzle and the internalcirculation, it was possible to establish gas contents of not more than10% by volume, based on the amount by volume of the gas in thehydrogenation bath, in the liquid phase of the reactor. Higher gascontents could not be achieved. By means of a reactor power balance, itwas possible to determine from the measurements that the mean bubblediameter in the liquid phase of the reactor was about 7 mm.

EXAMPLE 2

The procedure was as in example 1, but 0.1 m³ of nitrogen per 100 m³ ofhydrogen was additionally introduced via a bubbling line. The amount ofwaste gas of the reactor was regulated so that the purity of thedischarged hydrogen after removal of the steam by condensation was only90% by volume. There were no changes in the total pressure, the reactortemperature and the amount by weight of water in the hydrogenation bathin comparison with example 1. Assuming a completely back-mixed gasphase, the resulting total density of the gas in the reactor was thusabout 4.4 kg/m³, which is composed of the density of the nitrogen, ofthe hydrogen and of the steam. Measurements have subsequently shown thatthe gas content in the hydrogenation bath of the reactor increased,without further changes in the operation of the reactor, to 18% byvolume, based on the amount by volume of gas in the hydrogenation bath.This is attributable only to a reduction in the bubble sizes. A powerbalance of the reactor showed that the mean diameter of the gas bubblesin the hydrogenation bath after the addition of nitrogen had decreasedto about 4 mm.

EXAMPLE 3

A reactor as in example 1 was used. The fresh hydrogen was introducedinto the reactor with a purity of 99.99% by volume. With a total addedamount of 30 m³ (S.T.P.)/h of hydrogen, 29.997 m³ (S.T.P.)/h of hydrogenand 0.003 m³ (S.T.P.)/h of inert gases were thus actually introduced. Inorder to ensure a hydrogen purity of at least 99% by volume in the gasphase of the hydrogenation reactor, it was necessary to discharge 1%,based on the amount of fresh gas, of waste gas. The waste gas thencontained 0.297 m³ (S.T.P.) of hydrogen and 0.003 m³ (S.T.P.) of inertsubstances.

Starting from this operating state, the purity of the fresh hydrogen wasreduced to 99.9% by volume. The effort involved in the production of thehydrogen decreased considerably as a result. If, as previously, 1% ofwaste gas (based on the amount of fresh gas) was discharged, i.e. 0.3 m³(S.T.P.)/h left the reactor, this waste gas is composed of 0.27 m³ of H₂and 0.03 m³ of inert substances. Consequently, the hydrogen purity inthe waste gas after removal of the condensable fractions was about 90%by volume, which is in the particularly preferred range.

The advantage of this procedure is that less effort is involved in thepurification of the fresh hydrogen, slightly less hydrogen is lost andthere are smaller bubbles in the reactor, which provides a largerexchange surface.

EXAMPLE 4

The procedure was as in the initial state of example 3, but the amountof waste gas was reduced from 1% to 0.1%, based on the amount of freshgas, and the composition of the fresh hydrogen was not changed. Thus, inabsolute terms, only 0.03 m³/h of waste gas was discharged, said wastegas being composed of 0.027 m³/h of hydrogen and 0.003 m³/h of inertsubstances. The concentration of the hydrogen in the waste gas of thehydrogenation reactor was 90%.

The advantages of this variant are a substantially lower hydrogen lossand the larger mass transfer areas owing to smaller bubbles. The reactorrequires absolutely no further modifications for this purpose, and allthat is necessary is to further close the waste gas valve.

1. A process for hydrogenating liquid organic compounds selected from the group of mononitrobenzene, methylnitrobenzene, methylnitrotoluene, 2,4-dinitrotoluene and mixtures of 2,4-dinitrotoluene and 2,6-dinitrotoluene, wherein hydrogen present in the reactor contains proportions of at least one gas which is inert in the hydrogenation reaction, said gas being selected from the group of nitrogen, noble gases and ammonia, and wherein the sum of the proportions of the gases which are inert in the hydrogenation reaction is from 3 to 50% by volume, based on the gas phase in the reactor.
 2. A process as claimed in claim 1, wherein the sum of the proportion of the bases which are inert in the hydrogenation reaction is from 5 to 20% by volume, based on the gas phase in the reactor.
 3. A process as claimed in claim 1, wherein the gas which is inert in the hydrogenation reaction is fed to the reactor as a mixture with the hydrogen.
 4. A process as claimed in claim 1, wherein the gas which is inert in the hydrogenation reaction is fed into the reactor at a different point from the hydrogen.
 5. A process as claimed in claim 1, wherein the content of gas which is inert in the hydrogenation reaction is established by regulating the amount of waste gas discharged from the reactor.
 6. A process as claimed in claim 5, wherein the gas which is inert in the hydrogenation reaction is fed to the reactor as a mixture with the hydrogen.
 7. A process as claimed in claim 2, wherein the gas which is inert in the hydrogenation reaction is fed to the reactor as a mixture with the hydrogen.
 8. A process as claimed in claim 5, wherein the gas which is inert in the hydrogenation reaction is fed into the reactor at a different point from the hydrogen.
 9. A process as claimed in claim 2, wherein the gas which is inert in the hydrogenation reaction is fed into the reactor at a different point from the hydrogen.
 10. A process as claimed in claim 2, wherein the content of the gas which is inert in the hydrogenation reaction is established by regulating the amount of waste gas discharged from the reactor.
 11. A process for hydrogenating liquid organic compounds selected from the group of mononitrobenzene, methylnitrobenzene, methylnitrotoluene, 2,4-dinitrotoluene and mixtures of 2,4-dinitrotoluene and 2,6-dinitrotoluene, wherein hydrogen present in the reactor contains proportions of at least one gas which is inert in the hydrogenation reaction, said gas being selected from the group of nitrogen, noble gases and ammonia, and wherein the gas which is inert in the hydrogenation reaction is fed to the reactor as a mixture with the hydrogen.
 12. A process for hydrogenating liquid organic compounds selected from the group or mononitrobenzene, methylnitrobenzene, methylnitrotoluene, 2,4-dinitrotoluene and mixtures of 2,4-dinitrotoluene and 2,6-dinitrotoluene, wherein hydrogen present in the reactor contains proportions of at least one gas which is inert in the hydrogenation reaction, said gas being selected from the group of nitrogen, noble gases and ammonia, and wherein the gas which is inert in the hydrogenation reaction is fed into the reactor at a different point from the hydrogen.
 13. A process as claimed in claim 11, wherein the sum of the proportions of the bases which are inert in the hydrogenation reaction is from 5 to 20% by volume, based on the gas phase in the reactor.
 14. A process as claimed in claim 12, wherein the content of gas which is inert in the hydrogenation reaction is established by regulating the amount of waste gas discharged from the reactor. 